Method of making a heat-sensitive copysheet and resultant article



UllltCl METHOD F MAKING A HEAT-SENSITIVE COPY- SHEET AND RESULTANTARTICLE Filed Feb. 13, 1959, Ser. No. 7 92,985 12 Claims. (Cl. 117-36)This invention relates to the reproduction of graphic originals, such asprinted-or typewritten correspondence, sketches, drawings and the like,by thermographic copying methods involving brief irradiation of theoriginal with high .intensity radiation, and formation of a visiblereproduction of the resulting heat-image on a heat-sensitive copy-paperin heat-conductive pressure-contact therewith. The invention hasparticular reference to novel heat-sensitive copy-paper, in thepreferred examples of which the sensitizing materials includephosphomolybdate or equivalent complex salts.

The preferred sensitizing materials employed in making theheat-sensitive copy-papers of this invention are water-soluble, and cantherefore be applied to paper economically and safely in the form ofaqueous solutions. However the reactants may also be dispersed innonaqueous volatile liquid vehicles,V preferably together with suitablesoluble film-forming binders, and formed into visibly heat-sensitivefilms or coatings.

The preferred copy-sheets of this invention are based on-'complexphosphomolybdate salts and are white or very pale blue-green inappearance. They are stable at normal room and storage temperatures,under wide ranges of humidity, and when illuminated. When heated brieflyto temperatures of the order of 6Q-150 C., eg. by momentary contact witha heated metal bar, the copy-sheet rapidly converts to a deep bluecolor. The sheet produces clear and high-contrast reproductions oftypewritten or other graphic originals in the thermographic copyingprocess hereinabove identified. The areas corresponding to theradiation-absorptive printed States Patent yso Thereactants aredissolved in the water in the order named. Solution of thel magnesiumchloride results in .the formation of a voluminous white precipitate,which is removed by filtration before adding the remaining components.The resulting solution is found to be unstable, becoming dark blue incolor when allowedtoI stand at room temperature for 4-8 hours. It istherefore compounded and applied as rapidly as possible. Strips of thintranslucent paper (map overlay tracing paper) are dip-coated with thesolution and then dried rapidly in an air blast at not higher than 100F. The resulting sheet is white or very Vfaint blue in appearance,stable yat normal room conditions, and converted to dark blue by briefcontact with a metal bar at 1GO-120 C. The sheet is capable of providinga reproduction, by the thermographic copying process, of originalstypewritten on letterhead bond paper.

Substitution of ammonium vanadate for ammonium molybdate-in the formulaof Example l provides a copysheet which requires somewhat more extensiveheating for conversion to the colored form.

Example 2 A i Water 50 Ammonium molybdate 4 y Na2HPO4 Y. 1

Sodium citrate 1.5 Ascorbic acid 0.25

areas of the original are converted to a dense blue; the

background areas remain essentially unchanged. The sheet is sufficientlytransmissive of infra-red radiation to permit front-printing, in whichthe radiation.

may also be produced in accordance with the principles. of theinvention; but the normally white sheet producing a deep blue image ispreferred.

In the drawing, Figure 1 represents a heat-sensitive copy-sheet 12 infront-print position over a graphic original 10 having radiationabsorptive image areas 11, the composite being irradiated from a sourceof radiation 14 to produce colored image areas 13 in the heat-sensitivecopy-sheet. Figure 2 is a ow-sheet indicating the steps involved inproducing a typical heat-sensitive copy-sheet in accordance with theprinciples of theinvention.

The invention will now be further described in terms of specificillustrative but non-limitative examples in which all proportions are inparts by weight unless otherwise specified.

methods, the image areas appearing as a very deep blue on asubstantially White background. Y

The application of a very thin opacifying coat, e.g. of a smoothdispersion of titanium dioxide pigment in a solution of a polymericresin binder in a volatile organic solvent, increases the opacity of thesheet and results in increased contrast between image and backgroundareas in the finished reproduction. Thin surface coatings of this orother unpigmented transparent resinous or binder material are similarlyapplicable as protective coatings where desired. Y

The slight cloudiness of the sensitizing solution may be overcome byreducing the amounts of ammonium molybdate and sodium acid phosphate byone-half, but the image density is also reduced. Increasing theconcentration of reactants, eg. to double `the quantities shown,produces still more deeply but somewhat irregularly colored image areasin the copy; at these concen- The components arecombined asin thepreceding examples, and applied to 27 lb. opaque grcaseprooi ThilmanyNo. 908 paper, to provide a heat-sensitive copypaper useful in thethermographic reproduction of graphic originals. The linitially Whitesheet becomes a faint vpink on prolonged exposure to sunlight withoutdetracting from its ability to be rapidly converted to a deep blue onbrief heating at 10G-120 C.

Example 4 Water 50 Ammonium molyhdate 2 Na2SiO3 l Malic acid l 'lhiourea2 MgCl2-6H2O 12 Copy-paper prepared with this solution produces a blueimage with a light blue background. Stability of the sheet under roomand storage conditions is improved by addition of small amounts ofsodium citrate to the solution.

The firsttwo reactants are dissolved in the water at 180 F. VThesolution is then cooled to room temperature and the remainingingredients are added and dissolved. Paper is dip-coated in thesolution, dried, and tested as a heatsensitive copy-sheet. The initiallywhite sheet forms blue image areas on heating in the thermographic copyprocess.

Example 6 Tungstic acid, WO38H2O, 8 grams is dissolved in 200 ml. ofwater made slightly alkaline with sodium hydroxide and held at 80 C. Thesolution is brought to a pH of 6 with hydrochloric acid. There is thenadded 2 grams of disodium hydrogen phosphate and 48 grams of magnesiumchloride, and the solution is cooled to room temperature. To a 50 ml.portion there is added 3 grams of sodium thiosulfate and 0.25 gramsodium citrate.

T he solution is quicklyl applied to paper and dried, producing anormally stable copy-sheet which converts rapidly from a White to a blueappearance on heating to 10G-120 C. and produces useful copy bythermographic copying procedures.

Substitution of ascorbic acid for sodium thiosulfate in this formulacauses rapid conversion to a blue solution.

Molybdic oxide may similarly be dissolved by means of sodium hydroxideand the product used in place of the ammonium molybdate of Examples l-Sor the 'sodium tungstate of Example 6.

Example 7 Sodium vanadate, NaVO3, two parts, is dissolved in 50 parts ofwater with addition of sodium hydroxide and the solution brought to pH`6 with HC1 as in the preceding example.V The solution is reddishyellow. There is added one part of disodium acid phosphate and l2 partsof magnesium chloride. The solution is converted `to an orange solution.Three parts of sodium thiesulfate is then added, and the solutionapplied to paper and dried, producing a yellowish copy-paper whichconverts to blue at heated image areas in the thermographic copyingprocedure. Small amounts of sodium citrate may be included -to improvestability.

Example 8 An aqueous solution of ammonium molybdate, disodium acidphosphate and magnesium chloride in the proportions shown in Example 3is prepared and a portion is evaporated to dryness at room temperature.The solid residue is added to a solution of ethyl cellulose in acetone,and the mixture milled in a ball mill until smooth. Ascorbic acid insolution in alcohol is then added in the approximate proportionsemployed in lExample 3, and the mixture is coated on paper andrdried atroom temperature. The sheet is heat-sensitive, providing usefulreproductionsof graphic originals by the thermographic copying process.The sheet is highly stable both under normal room conditions and at highhumidities.

Another portion of the liquid mixture is spread on a smooth metal panel,permitted to dry, and then stripped from the metal surface .to provide aself-sustaining heatsensitive film product. The proportions of ethylcellulose and heat-sensitive reactants are selected to provide a strongflexible film with adequate image-forming properties.

Water-soluble bindei materials are similarly applicable. Thus,carboxymethyl cellulose, methylcellulose, polyvinyl pyrrolidoneandsimilar water-soluble film-forming materials are found useful inthickening aqueous solutions of these several reactant systems, to avoiddisplacement when applied to paper carrier webs at increased coatingthicknesses, and in forming self-supporting heat-sensitive iilms.

Example 9 Ammonium molybdate is dissolved in water. With the solutionboiling gently, aluminum chloride is cautiously added until a slightcloudiness begins to appear. The solution is then: cooled by placing inau ice bath, whereupon a White crystalline precipitate is formed whichis recovered.4 vThe precipitateis washed with ice-water and redissolvedin warm water. To the solution are then added zinc chloride, sodiumcitrate, and ascorbic acid in the approximate proportions indicatedinthe previous'examples, and the solution is applied to paper and dried.There is provided a heat-sensitiver copy-paper which converts fromWhiteto dark blue in the thermographic copying process and when brieflycontacted with a metal test bar heated to 10C-120 C.

The initial precipitation may alternatively'be accomplisned bysubstituting potassium alum Y (K230i A12 (S04) a 24H20) for the aluminumchloride.

Example 10 Example l] Theprecipitate formed by cooling a hot solution of2 parts of ammonium vmolybdate and one part of chrome alum is dissolvedin Warm Water. VSmall amounts of ascorbic acid, malic acid, and yzincchloride are added. Paper is impregnated with the solution and dried,forming Cautious addition of ferric ammonium sulfate solution to warmammonium molybdate solution produces a yellowish precipitate whichredissolves on continued stirring. Cooling of the solution then producesa white precipitate, which is recovered, washed, and re-dissolved at 42C. vSmall amounts of ascorbic acid and aluminum chloride are added andthe solution applied to paper and dried, producing a heat-sensitivecopy-sheet. The storage stability of `the sheet is improved by additionof sodium citrate.

Example 13 To 50 parts of hot water there is added 6 parts of ammoniummolybdate followed by 2 parts of cobalt chloride (CoCl26I-I2O) Areaction occurs, forming a reddish purple semi-coloidal dispersion. Tothis is added 8 parts of potassium persulfate. A greenish solutionresults, which is cooled in an ice bath. The greenish crystallineprecipitate formed is recovered on a filter, lightly rinsed withice-water, and redissolved in 50 parts of warm water. One part ofascorbic acid is added, and the solution4 used to saturate paper whichwhen dry is found to be useful as a heat-sensitive copypaper, formingblue-black heated image areas on a light blue-green background.

Analogous results are obtained by first combining the cobalt chlorideand potassium persulfate, then adding'the ammonium molybdate, andproceeding as indicated.

Example 14 treating solutions are used.

Example 15 To a colorless solution of four parts of ammonium molybdatein water there is added a faintly of one part of manganese sulfate(MnSO4-H2O) in water, forming a reddish-yellow solution of reactionproduct from which is obtained a yellowish residue on evaporation inglass at elevated temperatures. When the Solution is applied to paper orto aluminized paper or aluminum foil and dried at room temperature, theresulting sheet changes to a dark color on heating but requiresexcessive time and temperature of heating for effective use as aheatsensitive copy-sheet. Heat-sensitivity is improved by the additionto the coating solution of small amounts of starch or thiourea.

When combined in aqueous solution under the conditions indicated, themolybdate and phosphate compounds of Example 1 react to form aheteropoly complex phosphomolybdate anion. Analogous reactions occurwith the molybdate and silicate of Example 4, the molybdate and arsenateof Example 5, the tungstate and phosphate of Example 6, and the vanadateand phosphate of Example 7. Aluminum, chromium, iron, cobalt andmanganese likewise form heteropoly complex anions with molybdenum andthe like, as indicated in Examples 9-15. Similarly antimony, germanium,boron, copper,\tellurium, titanium, selenium, zirconium, thorium and thelike form oxygenated heteropoly complex anions with molybdenum, tungsten or vanadium. In these complex compounds the phosphorus or the likeprovides a central atom within an enclosing structure of molybdenum,tungsten or vanadium atoms and associated oxygen atoms. The availabilityof pink solution V the central atom is thereby reduced; for example,magnesium chloride docs not react with the' stable colorlessphosphomolybdate complex, whereas magnesium chloride and sodium acidphosphate in aqueous solution react readily, with precipitation of aninsoluble magnesium phosphate. The anion is also difiicultly reducible;but acidification of the heteropoly coplex results in polymerization andthe resulting onion is then found to be easily convertible to anintensely colored modification by reaction with a mild reducing agent.

Each of the several elements which provide the central atom or atomswithin the enclosing oxygenated structure of the heteropoly complexanion in the compositions and heat-sensitive copyheets of the foregoingexamples, i.e. elements such as phosphorus, silicon, arsenic, aluminum,manganese, and various others, may be shown to respond to the followingconditions:

a. The element in its ionic form has a positive valence of at least two.

`b. The element has an ionic radius not less than about 0.20 Angstromunit and not more than about 0.95 'Angstrom unit, as calculated fromcrystal structure data.

c. The ion is capable of adding doubly charged negative oxygen atoms toits coordination sphere in neutral or acid solution, i.e., is capable ofshowing acidic character.

In addition, the preferred elements in low concentration are non-toxicby absorption or inhalation.

Magnesium chloride is readily hydrolyzed, at the temperatures involvedinithermographic copying as herein described, to form hydrochloric acidand substantially water-insoluble magnesium hydroxide. Other equivalentwater-soluble hydrolyzable salts of strong acids, effective in thecopy-sheet compositions of this invention, include aluminum chloride,calcium chloride, cadmium chloride, lithium chloride, magnesium bromide,magnesium nitrate, magnesium sulfate, manganese chloride, manganesesulfate, nickel chloride, strontium chloride, zinc chloride, and zincsulfate. On the other hand, barium chloride does not hydrolyze, thesalt-forming reaction being dominant and barium hydroxide being highlywater-soluble at the copying temperature. The same is true of the sodiumand potassium salts of strong acid's. These salts are ineffective forthe purposes of the invention. Again, salts of metals such as mercury,tin and antimony, which in aqueous solution provide easily oxidizablemetal cations, cannot effectively be introduced in thesensitizing'solution without immediate inter-reaction and are thereforeto be avoided for the present purposes. Strong-acid salts of metals suchas iron, chromium, and cobalt, in which the metal ion is not readilyoxidizable, have been used, but are themselves strongly colored and tendto reduce the contrast obtainable in the resulting copy, and hence areless desirable than the colorless or lightly colored salts. Calciumnitrate has been usefully substituted for magnesium chloride in theseformulations but the resulting sheet'is somewhat less stable on storageand this type of compound, like the strongly colored salts, is notordinarily preferred. e

Stability of the copy-sheet involves the ability of the sheet to remainrapidly visibly reactive on prolonged storage. The sheet must notdiscolor or darken to any appreciable extentl and the color-formingreaction must not occur prematurely. Many of the compositions heredescribed produce copy-sheets having adequate stability for manypurposes even in the absence of specic stabilizer materials; however itis generally found desirable to further stabilize the compositions bythe addition of a stabilizer, such for example as a water-soluble saltof a weak acid having an ionization constant of about l03 to l0F1 oreven somewhat lower. Preferred examples include ammonium or alkali metalcitrates, tartrates, acetates, formates, lactates, benzoates, phthalatesand malates. Carbonates and bicarbonates are less effective but stilluseful.

Ingeneral, water-soluble reducing agents whichy exhibit a mild reducingaction in acid solution are useful in these heat-sensitive copy-sheets.Ascorbic acid is a preferred material in many of these compositions.With ascorbic acid as the sole reducing agent, the composition willordinarily require increased amounts of stabilizer. Ascorbic acid isalso expensive. On the other hand, thiourea is relatively inexpensiveand sheets made'with this material are stable with much lower amounts ofstabilizer; but higher activation temperaturesare then conditions, andto produce excellent images at readily available temperatures. Othermild reducing agents which have been found useful, either alone or incombination, as components of the heat-sensitive copy-sheets of thisinvention include itaconic acid, malic acid, Elon(para-aminophenolsulfate), and tartaric acid.

In some cases, extremely mild reducing agents are found to produce onlya slight visible change on heating of the copy-sheet. Sodium thiosulfatein the modified formula of Example 2 provides one such formula. On theother hand, strong reducing agents, and particularly actions. Onheating, acid is first liberated, by hydrolysis of magnesium chloride inthe presence of liberated Water or water vapor. The presence of acidcauses rearrangement of the phosphomolybdate or equivalent complex fromthe initially stable form to a readily reducible modification. In someinstances the acid also presumably imparts additional activity to thereducing In any event, at the temperatures inand regardless of theory,before indicated are found to handling,

central atom within an enclosing structure of atoms selected from theclass consisting of molybdenum, tungsten acid solution.

2. AA heat-sensitive copy-sheet suitable for the thermographicreproduction of graphic originals, stable under nese sulfate, nickelchlorlde, strontium chloride, zinc chloride, and zinc sulfate; awater-soluble reducing agent having a mild reducing action in acidsolution; and, as a stabilizer material, a water-soluble salt of a weakorganic acid having an ionization constant not greater than about 104.

3. A heat-sensitive lected from the class consisting of molybdenum,tungsten and vanadium, and mixtures thereof, a Water-soluble aluminumlithium chloride, magnesium bromide,lmagnesium chlo- S. The copy-paperof claim 3 in which the paper support member is coated with theheat-sensitive layer, the latter including a nlm-forming binder.

6. A heat-sensitive copy-paper yas defined in claim 3 and including aprotective heat-stable continuous tilmlike surface coating.

7. A stable heat-sensitive copy-paper suitable for the thermographicreproduction of black-on-whitegraphic perature Within the range of about60l5 0 C., comprising a thin flexible paper support member carrying avisibly heat-sensitive layer consisting essentially of water-solublediiiiculty reducible phosphomolybdate complex heteropoly compound,magnesium chloride, ascorbic acid, and sodium citrate.

8. A heat-sensitive copy-sheet suitable for the thermographicreproduction of graphic originals, stable under storage and handlingconditions, and capable of permanently converting to a Vvisibly distinctform on being momentarily pressed against a metal test bar at atemperature within the range of about 60-150" C., comprising a papersupport member and a thin, stable, visibly heat-sensitive coating of afilm-forming Waterinsoluble polymeric organic binder containing,uniformly dispersed therein, components comprising: a water-solublecompound of a cation and a diicultly reducible heteropoly complexoxygenated anion having a central atom within an enclosing structure ofatoms selected from the class consisting of molybedenum, tungsten andvanadium, and mixtures thereof; a water-soluble hydrolyzable saltselected from the class consisting of aluminum chloride, calciumchloride, cadmium chloride, lithium chloride, magnesium bromide,magnesium chloride, magnesium nitrate, magnesium sulfate, manganesechloride, manganese chloride, manganese sulfate, nickel chloride,strontium chloride, zinc chloride, and zinc sulfate; and a water-solublereducing agent having a mild reducing action in acid solution.

9. A stable heat-sensitive copy-paper suitable for the thermographicreproduction of black-on-white graphic originals as blue-on-whitecopies, capable of permanently converting from a white to an intenseblue form on being momentarily pressed against a metal test bar at atemperature within the range of about (S-150 C., comprising a thinexible paper support member carrying a visibly heat-sensitive layerconsisting essentially of a water-soluble diicultly reduciblealuminomolybdate complex heteropoly compound; aluminum chloride; awater-soluble reducing agent having a mild reducing action in acidsolution; and, as a stabilizer material, a water-soluble salt of a weakorganic acid having an ionization constant within the range of about -3to 10-7.

10. The method of making a heat-sensitive copy-sheet suitable for thethermographic reproduction of graphic originals and capable ofpermanently converting to a visibly distinct form on being momentarilypressed against a metal test bar at a temperature within the range ofabout 60150 C., comprising: coating on a receptive thin exible supportweb a liquid composition comprising a volatile liquid vehicle, awater-soluble compound of a cation and a diicultly reducible heteropolycomplex anion having a central atom within an enclosing structure ofatoms selected from the class consisting of molybdenum, tungsten andvanadium, and mixtures thereof, a water-soluble hydrolyzable saltselected from the class consisting of aluminum chloride, calciumchloride, cadmium chloride,

lithium chloride, magnesium bromide, magnesium chlo- 10 ride, magnesiumnitrate, magnesium sulfate, manganese chloride, manganese sulfate,nickel chloride, strontium chloride, zinc chloride, and zinc sulfate,and a watersoluble reducing agent having a mild reducing action in acidsolution; and drying said coating while avoiding interreaction of saidcomposition.

l1. A stable heat-sensitive copy-paper suitable for the thermographicreproduction of black-on-White graphic originals as blue-on-whitecopies, capable of permanently converting from a White to an intenseblue form on being momentarily pressed against a metal test bar at atemperature within the range of about -150 C., comprising a thinflexible paper support member carrying a visibly heatsensitive layerconsisting essentially of Water-soluble difficultly reduciblephosphomolybdate complex heteropoly compound, magnesium chloride, awater-soluble reducing agent having a mild reducing action in acidsolution, and, as a stabilizer material, a water-soluble salt of a Weakorganic acid having an ionization constant within the range of about 103to 10J'.

12. A heat-sensitive copy-sheet suitable for the thermographicreproduction of graphic originals, and capable of permanently convertingto a visibly distinct form on being momentarily pressed against a metaltest bar at a temperature within the range of about 60-150 C., saidcopysheet including a visibly heat-sensitive layer comprising: awater-soluble compound of a cation and a diffcultly reducible heteropolycomplex oxygenated anion having a central atom Within an enclosingstructure of atoms selected from the class consisting of molybdenum,tungsten and vanadium, and mixtures thereof; magnesium chloride;Water-soluble reducing agent having a mild reducing action in acidsolution; and, as a stabilizer material, a water-soluble salt of a weakorganic acid having an ionization constant within the range of about10-3 to 10"'.

Miller et al. Dec. 23, 1953 Miller Apr. 3. 1956

1. A HEAT-SENSITIVE COPY-SHEET SUITABLE FOR THE THERMOGRAPHICREPRODUCTION OF GRAPHIC ORIGINALS, CAPABLE OF PERMANENTLY CONVERTING TOA VISIBLY DISTINCT FORM ON BEING MOMENTARILY PRESSED AGAINST A METALTEST BAR AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 60-150*C., ANDINCLUDING A VISIBLY HEAT-SENSITIVE LAYER COMPRISING: A WATER-SOLUBLECOMPOUND OF A CATION AND A DIFFICULTLY REDUCIBLE HETEROPOLY COMPLEXOXYGENATED ANION HAVING A CENTRAL ATOM WITHIN AN ENCLOSING STRUCTURE OFATOMS SELECTED FROM THE CLASS CONSISTING OF MOLYBDENUM, TUNGSTEN ANDVANADIUM, AND MIXTURES THEREOF; A WATER-SOLUBLE HYDROLYZABLE SALTSELECTED FROM THE CLASS CONSISTING OF ALUMINUM CHLORIDE, CALCIUMCHLORIDE, CADMIUM CHLORIDE, LITHIUM CHLORIDE, MAGNESIUM BROMIDE,MAGNESIUM CHLORIDE, MAGNESIUM NITRATE, MAGNESIUM SULFATE, MANGANESECHLORIDE, MANGANESE SULFATE, NICKEL CHLORIDE, STRONTIUM CHLORIDE, ZINCCHLORIDE, AND ZINC SULFATE; AND A WATERSOLUBLE REDUCING AGENT HAVING ANDMILD REDUCING ACTION IN ACID SOLUTION.